1. Field of the Invention
The present invention relates to a pipe trap. More particularly, the present invention relates to a pipe trap for a chemical vapor deposition system.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a thin film deposition technique for forming a layer over a chip. In the CVD process, chemical reactants (in general, reactive gases) react inside a reaction chamber (or furnace) to form solid product on chips. Chemical vapor deposition has a wide application in the fabrication thin films on semiconductor devices. In general, conductive, semi-conductive or dielectric films are fabricated by performing chemical vapor deposition processes. Furthermore, because the material for forming the thin film layer in a chemical vapor deposition is formed by reacting reactive gases, both the crystallinity and stoichiometry of the thin film layer are better than one formed by a conventional sputtering method. Hence, chemical vapor deposition has become the principal thin film deposition tool in advance semiconductor production facility.
However, when the reactive gases react to form solid material in a chemical vapor deposition process, a large amount of reactive material particles and byproducts is also produced. Thus, the reaction chamber (or furnace) is usually linked to a disc trap for filtering the reactive material particles and byproducts within the gaseous exhaust.
FIG. 1 is a section view showing the structure of a conventional disc trap linked to the reaction chamber (furnace) of a chemical vapor deposition system. As shown in FIG. 1, the disc trap 100 mainly comprises a disc trap body 110, a filtering plate 120 and a disc filter 130. The disc trap body 110 further comprises a base 112, a tube body 114 and a fixed shaft 116. The tube body 114 is set up over the base 112. The tube body 114 has a gas inlet 114a and a gas outlet 114b. The fixed shaft 116 is set up on the base 112. The filtering plate 120 is set up inside the tube body 114 facing the gas inlet 114a. The filtering plate 120 has a plurality of pores (not shown) for filtering reactive material particles within the gaseous exhaust. The disc filter 130 comprises a plurality of ring-shaped discs 132. The ring-shaped discs 132 are stacked together on top of the base 112 to form a hollow tube.
When exhaust containing reactive material particles and reaction byproducts enter into the disc trap body 110 via the gas inlet 114a, a portion of the gases will pass through the filtering plate 120 into the disc filter 130 while the remaining portion will pass directly into the disc filter 130. For the gases that pass through the filtering plate 120, reactive material particles having a size greater than the pores in the filtering plate 120 are trapped. Furthermore, there are gaps between the discs 132 of the disc filter 130. Therefore, the disc filter 130 is able to trap reactive material particles in the gases having a size bigger than the gap so that most reactive material particles or impurities are removed when the gases exhaust from the gas outlet 114b. 
However, the conventional disc trap has the following disadvantages:
1. The exhaust gases entering the disc trap from the gas inlet travel directly to the disc filter so that the gas flow path is rather short. Since it is difficult to retain the reactive material within the exhaust within the disc trap for a longer period, the filtering efficiency is low.
2. Although the disc filter has the capacity to hold back reactive material particles having a relatively large diameter, reactive material particles having a smaller diameter are free to go. Hence, the disc trap is a poor filter for small particles.
3. The filtering plate is adjacent to the gas inlet of the disc trap. In fact, the filtering plate and the gas inlet separate from each other by 1.5 cm only. Furthermore, the pores on the filtering plate are relatively small. If the exhaust gases contain a large quantity of reactive material particles and byproducts, some of the pores may be blocked by the particles and hence the gas inlet is choked. When the gas inlet of the disc trap is choked, time and money must be spent to decongest the inlet. Ultimately, productivity of the chemical vapor deposition system will drop.